Apart from the increased security, vehicle checkpoints in midtown Manhattan and armed guards in front of the Office of Emergency Management, no one was in any doubt that the latest terror threats were being taken very seriously.

The 10th September 11 anniversary has a different feel.

Police cruisers took up positions on midtown blocks on Lexington, Park and Fifth avenues as officers stopped trucks and other vehicles for inspection.

Drivers were made to open the storage spaces of delivery trucks. In Brooklyn, US marshals armed with machine guns guarded the federal courthouse and increased security was observed in front of the nearby city emergency management office.

"We have already had a full complement of people working shifts because of the September 11 anniversary prior to this," said Jim Margolin, a spokesman for the Federal Bureau of Investigation's New York office. "We are taking the logical investigative measures to assess this threat."

New Jersey's Director of Homeland Security Charles McKenna said state officials were concerned that as New York and Washington were "hardened targets", the would-be attackers might turn to New Jersey as a back-up option.

Mr McKenna said the state has increased security. He declined to specify what steps have been taken.

New Jersey Governor Chris Christie said people should not alter their plans and lives.

"This is nothing that should come to folks as any surprise nor is it something that should cause panic," Mr Christie said.

On September 11, 2001, terrorists hijacked four airliners. Two were flown into the twin World Trade Centre towers in New York; a third hit the Pentagon in Arlington, Virginia; and one crashed in Shanksville, Pennsylvania, after passengers attacked the hijackers. Close to 3,000 people were killed.

New York police have stopped at least 13 terrorist attacks since 2001, Mayor Michael Bloomberg said.

While the killing of al-Qa'ida leader Osama bin Laden has helped reduce the threat, the mayor said: "The one thing we know is the terrorists have not gone away."

In February 2010, Colorado resident Najibullah Zazi pleaded guilty in Brooklyn federal court to a plot to detonate bombs on New York subways around the anniversary of the September 11 attacks.

The Brooklyn federal courthouse has supplemented its usual security, said James Elcik, supervisory deputy US marshal.

Threat

"We're here to protect the building, and more importantly the people in it, around the anniversary of 9/11 given the non-specific threat," Mr Elcik said, standing next to an armed guard on Adams Street, where tourists and bicyclists entered the pedestrian path over Brooklyn Bridge.

He said it hadn't been decided whether the extra security would also be in place Monday and throughout next week.

At the subway station in the Port Authority bus terminal at 42nd Street, two police officers stood guard at the turnstiles while two others patrolled the platform. Police officers in a group of four stood on the platform at Broadway and Murray streets in Lower Manhattan giving directions to tourists making their way to and from Ground Zero, four blocks south.

"We have threats all the time," Mr Bloomberg said yesterday. "Each time we increase our security, which obviously we had done for this. Are we increasing a little more? Yes, we're increasing a little more but there's a limit to how much you can have, just because you can't have a cop on every corner. But remember, a lot of the precautions we take, you don't see."